Cyanobacteria have evolved an auxiliary light-harvesting system, the phycobilisome (PBS) that allows absorption of sunlight, primarily in the 575-675 nm region, and unidirectional excitation energy transfer toward the chlorophyll-pigment bed of PSII reaction centers. Each phycobilisome has two main structural parts, the core-cylinders and the peripheral rods. Core cylinders are made of allophycocyanin (αβ)3 discs stacked next to each other. The core cylinders axis is parallel to the thylakoid membrane surface with at least two of the cylinders resting with their long axes on the stromal side of the thylakoid membrane. These provide a structural and excitation energy transfer link to the chlorophyll-pigment bed of PSII reaction centers (Elmorjani et al. 1986; Glazer and Melis 1987; Ducret et al. 1996; Glazer 1989; Arteni et al. 2009). In Synechocystis sp. PCC 6803 (Synechocystis), there are three allophycocyanin core cylinders, two of which rest directly onto the thylakoid membrane. A third cylinder is resting on the stromal side of the furrow formed by the other two core cylinders (Arteni et al. 2009). Core cylinders contain the pigment-proteins allophycocyanin-α and allophycocyanin-β encoded by the APCA and APCB genes and a small linker polypeptide LC, encoded by the APCC gene (Grossman et al. 1993; Glazer 1998; MacColl 1998). They are linked to the thylakoid membrane and the PSII dimer chlorophyll-pigments by a PBS terminal excitation-acceptor allophycocyanin pigment containing the linker polypeptide LCM, encoded by the APCE gene (Houmard et al. 1990; Ajlani and Vernotte 1998). The latter functions together with the products of the APCD and APCF genes to facilitate efficient excitation energy transfer from the phycobilisome toward the PSII reaction center (Ashby and Mullineaux 1999; Barber et al. 2003; Mullineaux 2008). Peripheral to the allophycocyanin core cylinders are phycocyanin-containing rods, also in cylinder form, physically extending outward from the allophycocyanin core cylinders (Glazer and Melis 1987; Glazer 1998; Arteni et al. 2009). Similar to the allophycocyanin, the phycocyanin rods are composed of stacked discs, each one made by six hetero-dimers of the pigment containing CPC-α and CPC-β proteins, encoded by the CPCA and CPCB genes respectively (Grossman et al. 1993; Glazer 1998; MacColl 1998). The CPC-α and CPC-βdimers are connected by linker polypeptides, encoded by CPCC1, CPCC2, and CPCD genes (Grossman et al. 1993; Ughy and Ajlani 2004). Genes CPCA, CPCB, CPCC1, CPCC2 and CPCD are clustered in a single operon in Synechocystis, which is referred to as the C-phycocyanin (CPC)-operon.
The phycobilisome substantially increases the sunlight absorption cross-section of PSII (Glazer and Melis 1987; Glazer 1989), thereby countering a potential imbalance in excitation energy distribution due to the high PSI/PSII stoichiometric ratio in cyanobacteria (Melis and Brown 1980, Myers et al. 1980), and the fact that most of the chlorophyll is associated with PSI in these microorganisms (Manodori et al. 1984; Glazer and Melis 1987). Up to 450 phycocyanin (PC) and allophycocyanin (AP) pigments can be associated with the PBS in Synechocystis. This large light-harvesting antenna size confers a survival advantage in the wild, where cells grow under light-limiting conditions. Under direct sunlight, however, the rate of photon absorption far exceeds the rate with which photosynthesis can utilize them, and excess light-energy is dissipated by non-photochemical quenching (Müller et al. 2001, Kirilovsky 2007; Bailey and Grossman 2008; Kirilovsky and Kerfeld 2012). A soluble carotenoid binding protein (orange carotenoid protein, OCP) plays essential role in this process in Synechocystis. The quenching of maximal fluorescence increases from 25-30% in WT cells to 60%-70% in cells overexpressing the OCP (Kirilovsky and Kerfeld 2012). Wasteful dissipation of excess absorbed irradiance would result in a suboptimal sunlight energy conversion. Moreover, dissipation of excess absorbed energy would enhance the probability of photodamage and photoinhibition of photosynthesis (Melis 1999). As a result, the utmost measured sunlight-to-biomass energy conversion efficiencies of cyanobacterial photosynthesis were reported to be in the range of 1-2%, whereas the theoretical maximum is 8-10% (Melis 2009). This pitfall affects all photosynthetic organisms (Melis 2009). It was alleviated in green microalgae, upon minimizing the size of the chlorophyll light-harvesting antenna, effectively limiting the capacity of the photosystems to absorb sunlight. This prevented over-absorption of photons by individual cells, enabling deeper sunlight-penetration into the culture, and affording an opportunity for more cells to be productive, in effect raising photosynthetic productivity of the culture as a whole (Polle et al. 2003, Kirst et al. 2012a, Kirst et al. 2012b). This concept of increasing photosynthetic productivity of a mass-culture under direct sunlight upon minimizing the light-harvesting antenna size is known as the Truncated Light-harvesting Antenna (TLA) concept (Melis 2009; Melis 2012; Kirst and Melis 2014).
However, applicability of the TLA concept in photosynthetic cyanobacteria has been questioned. Indeed, it was reported that a targeted truncation of the phycobilisome light-harvesting antenna of the model cyanobacteria Synechocystis lowered, rather than increased, productivity (Page et al. 2012). This is the opposite to the substantially increased the productivity of TLA plants and algae. (See also Liberton et al 2013). Applicants have now discovered that TLA can be applied to cyanobacteria to increase photosynthetic productivity.